Processing and Development of an Ultra-Light, High Strength Material through Powder Metallurgy

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Title: Processing and Development of an Ultra-Light, High Strength Material through Powder Metallurgy
Author: Neville, Brian Patrick
Advisors: William Roberts, Committee Member
Jay Tu, Committee Member
Carl Koch, Committee Member
Afsaneh Rabiei, Committee Chair
Abstract: The development of metal foams has produced materials with improved properties as compared to non-metal foams and solid metals. Metal foams can be classified as either open cell or closed cell. Open cell foam can be thought of as a network of interconnected solid struts, which can allow a fluid media to pass through it. A closed cell foam is made up of adjacent sealed pores with shared cell walls. Metal foams offer higher strength to weight ratios, increased impact energy absorption, and a greater tolerance to high temperatures and adverse environmental conditions when compared to bulk materials. The energy absorbing capability is one of the most interesting properties of closed cell foams. Under compression, these foams plastically deform over a large region of strain, often exceeding 50%, until the cells have completely collapsed and the material begins to behave as a bulk material Metal foams have been created through various processes that introduce a gaseous phase, either by physical or chemical means, into a molten metal or a powdered metal compact. Due to imperfect production techniques and lack of precise control over the formation of the cell structure, the mechanical properties of metal foams are often unpredictable. The cellular structure in closed cell metal foams is typically disordered, due in part to variation in the thickness of the cell walls and non-uniform shape and size of the cells. This leads to a variation in the cellular structure across the bulk material, creating varying mechanical properties. In this study, a new closed cell composite metal foam has been processed for the first time using powder metallurgy and characterized. The foams are processed by filling the vacancies between randomly ordered, densely packed, preformed hollow spheres with a metal powder and sintering them into a solid structure. Several different foams have been produced, using different sized hollow spheres and different sphere and matrix materials. The density of the foams have been measured and compared to the theoretical density calculated using an equation developed for these foams. These materials have undergone hardness and microhardness testing, static compression testing, and the microstructure has been observed on chemically etched and unetched samples through optical microscopy, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy. Compression-compression fatigue testing has been performed based on the plateau strength calculated from the results of static compression testing and the results have been evaluated The strength of the newly developed composite foam averaged 127 MPa from the yield point up to 54% strain, which is a strength eight times greater than that of the best reported results to date. The value for energy absorption is 68 MJ⁄m³ at 50% strain, which is seven times greater. Although denser than foams made solely from hollow spheres, the new composite metal foam developed in this study displays superior compressive strength and energy absorption capability, while exceeding strength to density ratios, as compared to the next best existing metal foam, processed using similar materials, but through a different technique. The combination of these properties gives opportunity for use in several applications where light weight, high stiffness and energy absorbing capability are required, such as automobile crumple zones, structural members in air, naval, and space craft, and in biomedical prosthesis.
Date: 2007-06-23
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/4133


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